Study On The Kinetic And Thermodynamic Characteristics Of Methane Hydrate Formation

Excessive carbon dioxide emissions lead to global warming,endangering the survival of all mankind.Many countries have proposed long-term goals of carbon neutrality or net zero emissions after the Paris Agreement.As of May 2022,countries have proposed carbon neutrality targets and climate neutrality commitments by incorporating national laws and policy oaths.China regards carbon peak and carbon neutralization as the key work in the 14 th Five-Year Plan period.Natural gas has attracted wide attention due to its high calorific value,low pollution,large reserves and wide distribution.The existing natural gas storage and transportation methods are mainly divided into gas storage and transportation and liquid storage and transportation,both of which have potential safety hazards of natural gas leakage.In order to improve the shortcomings of traditional natural gas transportation methods and reduce the leakage of natural gas during transportation,hydrate technology was applied.In this paper,the molecular dynamics method was used to study the effects of SiO₂ gap,SDS,CTAB and metal nanoparticles on the kinetic and thermodynamic characteristics of methane hydrate formation and decomposition process.The kinetic characteristics of methane hydrate in homogeneous solution were characterized by mean square displacement,diffusion coefficient,radial distribution function,molecular cluster growth curve and density distribution cloud map.The thermodynamic properties of methane hydrate in homogeneous solution were characterized by the changes of binding energy and thermal conductivity.The main research work of this paper is as follows:（1）The optimal conditions for the formation of methane hydrate and the formation of methane hydrate in the SiO₂ gap were studied.The effects of temperature,pressure and SiO₂ on the kinetic and thermodynamic properties of methane hydrate were studied by molecular dynamics method.It is found that the homogeneous solution at 275K and 15 MPa is most conducive to the growth of methane hydrate.Compared with the complete SiO₂ system,the methane hydrate synthesis area in the defective SiO₂system is more dispersed and evenly distributed.The thermal conductivity changes of homogeneous solution and homogeneous solution in SiO₂ gap are calculated by EMD method and NEMD method.Both methods can conclude that the existence of SiO₂ gap can reduce the thermal conductivity of homogeneous solution.（2）The promotion mechanism of SDS and CTAB molecules on the formation of methane hydrate was studied.The results show that the hydrate formation rate is the fastest in the SDS solution with a mass fraction of 1.2%.When the number of molecules in the homogeneous solution is the same,the SDS molecules have a better effect on hydrate formation.The adsorption effect of SDS molecules on methane molecules in 0.9%SDS solution is stronger than that of CTAB molecules in 1.2%CTAB solution.When the concentration of the two accelerators increases,the adsorption effect of SDS molecules on methane molecules is weaker than that of CTAB molecules.Considering the formation rate and gas storage density at the same time,SDS solution and CTAB solution with a mass fraction of 1.2%are most conducive to hydrate formation and methane gas storage.（3）The effects of metal nanoparticles on the structural changes of methane hydrates,including the formation and decomposition of methane hydrates,were studied.The NEMD method was used to calculate the change trend of thermal conductivity of metal nanoparticle system with time.The calculation results show that the thermal conductivity of the0.5nm metal nanoparticle system is close to the thermal conductivity of the homogeneous solution at 275K;when the particle size increases to 1.0nm,the thermal conductivity increases,which is not conducive to the formation and storage of methane hydrate.By analyzing the binding energy and density distribution,it is found that the addition of 0.5nm nano-Cu particles is beneficial to the heterogeneous nucleation of methane hydrate,while the surface of nano-Fe particles is not conducive to the nucleation of methane hydrate.The decomposition of methane hydrate by 2.0nm and 3.0nm metal particles was studied.The results show that the addition of metal nanoparticles can significantly accelerate the decomposition of methane hydrate.Small particle size metal nanoparticles are more likely to destroy the hydrate structure;compared with the type of metal particles,the particle size has a greater influence on the structure of methane hydrate.The research content of this paper can provide theoretical support for improving the formation rate and gas storage density of methane hydrate,and provide reference for the selection of methane hydrate formation accelerator,which is of great significance for the stable transportation of natural gas.